1. Field
The invention relates to apparatus and methods for sampling, particularly high resolution qualitative and quantitative microanalysis for liquid chromatography, gas chromatography and mass spectrometry.
2. State of the Art
Both manual and automated sampling apparatus and sampling methods for gas chromatography, liquid chromatography and mass spectrometry have been utilized in the prior art. The typical manual prior art sampling apparatus and sampling method are through the use of a syringe to inject a sample through a septum into a flow of carrier material, which then transports the sample to the analytical instrumentation. The typical automated prior art sampling apparatus and sampling method are through the use of a sample loop and valve. In the simplest form, the valve is designed such that it has three separate flow pathways. In the nonsampling position, the valve is set such that a carrier material is pumped from a carrier material source through the first pathway to a detector, and the sample is pumped from a sample source through the second pathway into a sample loop and then into the third pathway, and finally exiting to a sample vent. After the sample loop is completely flushed and full of sample, the valve is rotated to the sampling position which rotates the pathways such that the first pathway connects the carrier material supply with one end of the sample loop, the second pathway connects the other end of the sample loop with the detector, and the third pathway connects the sample source to the vent. In this mode the carrier material then flows through the first pathway and into the sample loop and pushes the sample out of the loop into the second pathway and then to the detector. The third pathway allows for the bypass of sample from the sample source to the vent. After the sample volume is pushed from the loop by the carrier material to the detector, the valve is rotated back to i its initial nonsampling position to flush the carrier material from the sample loop and refill the sample loop with sample. This process is repeated for multiple sampling. In more complex forms, there are many arrangements of the valves and the pathways to provide variations for multiplicity of samples and loops such that sampling loops may be filled simultaneously and the valve rotated through a multiplicity of sampling positions. Additionally, there are combinations which allow for flushing and backflushing of the valve, pathways and sampling loop between sampling.
The sampling apparatus and sampling methods of the prior art exhibit many limitations, especially when applied to microanalytical techniqes. The prior art apparatus requires a valve with moving parts and pathways through which sample flows. When switching the direction of flow between various pathways, leaks may develop if good seals are not made, or the moving parts may be subject to damage by foreign matter which may be introduced into the valve with the sample or carrier material. If the sample consists of a corrosive material, the corrosive material may also damage the sampling apparatus. Furthermore, the valves are not such that they can be made of the same inert materials of which the detector into which the sample is being injected is made. Therefore, portions of the sample may be absorbed by the sampling apparatus or foreign material may be leached out of the sampling apparatus into the sample.
Because of these construction material limitations, prior art sampling apparatus often have an upper operating temperature of approximately 250.degree. C. At temperatures beyond 250.degree. C. the materials used in constructing the sampling apparatus may be damaged. Since the size of the sample loop is typically set to a specific length of tubing, the sample volume is not easily varied. The amount of sample required to flush the sample loop and fill it requires a relatively large amount of sample volume, and results in relatively long sampling cycles.
The relatively long sampling cycles result in the sample being in extended contact with the carrier material during which diffusion between the sample and carrier material occurs. This perturbation of the sample results in the reduction in the qualitative and quantitative resolution of sample components.